1. Field of the Invention
The present invention relates to a pressure sensitive conductive sheet used mainly for the operation of various electronics, as well as a panel switch using the same.
2. Description of the Related Art
In recent years, as the functions and types of electronics, such as cellular phones and car navigation systems, have increased, various types of panel switches which make operation possible without failure have been in demand, as panel switches used for the operation of such electronics.
These conventional panel switches are described with reference to FIGS. 8 and 9. Here, in the cross sectional diagram among these figures, the configuration is shown with the dimensions enlarged in the direction of the thickness for ease of understanding.
FIG. 8 is a cross sectional diagram showing a conventional panel switch. In FIG. 8, pressure sensitive conductive sheet 1 is formed of base 2, for example of insulating silicone rubber. Base 2 has an indeterminate form with carbon or the like dispersed in silicone or the like, and conductive particles 3 having a size of 20 μm to 200 μm are dispersed in this. Substrate 4 is placed on the lower surface of pressure sensitive conductive sheet 1, and plurality of fixed contacts 5A and 5B made of silver or carbon are formed on the upper surface thereof. In addition, spacer 6 is formed of an insulating resin in such a manner as to surround fixed contacts 5A and 5B between pressure sensitive conductive sheet 1 and substrate 4. Thus, a panel switch is formed of the lower surface of pressure sensitive conductive sheet 1 and fixed contacts 5A and 5B, which face each other.
The thus formed panel switch is mounted in an operation portion of an electronic in such a manner that plurality of fixed contacts 5A and 5B are connected to an electronic circuit (not shown) in the electronic via lead wires (not shown) or the like.
In the above described configuration, when the upper surface of pressure sensitive conductive sheet 1 is pressed through an operation, pressure sensitive conductive sheet 1 bends downward, so that the lower surface makes contact with fixed contacts 5A and 5B. Then, conductive particles 3 inside base 2 approach and make contact with each other, when compressed through the pressing force, and thus, fixed contact 5A and fixed contact 5B are electrically connected via these conductive particles 3.
FIG. 9 is a graph showing the characteristics of the resistance of a conventional panel switch. As shown by curve A in FIG. 9, the number and contact area of conductive particles 3 which make contact with each other inside base 2 increase and the resistance value becomes smaller when additional pressing force is applied. Thus, the electronic circuit detects this electrical connection and change in the resistance value, so that operation of electronics having various functions becomes possible using this configuration.
Pressure sensitive conductive sheet 1 used in such a panel switch is formed in such a manner that a predetermined resistance value can be gained when base 2 is elastically deformed through a pressing force, and thus, conductive particles 3 inside base 2 are made to make contact with each other, as described above, and therefore, fluctuation in the resistance value resulting from repeated operation is great. As shown in FIG. 9, in some cases, change in the resistance value shifts from curve A to curve B after several thousands of pressing operations.
In some cases, for example, the resistance value changes by almost one digit, depending on the pressing force, so that resistance value R1 of several tens of kΩ when pressing force P of 2 N to 10 N is applied changes to resistance value R2 of several kΩ for the same pressing force P after the pressing operation has been repeated. Therefore, it is necessary to set the electronic circuit for detecting change in the resistance value taking into account such change.
In addition, pressure sensitive conductive sheet 1 is formed by dispersing conductive particles 3 inside base 2 made of silicone rubber or the like, which easily deforms, and a certain thickness is required. Usually a sheet having a thickness of approximately 0.3 mm to 1 mm is used. Here, Unexamined Japanese Patent Publication No. 2006-236988, for example, is known as a prior art document relating to the invention of the present application.
In the above described conventional panel switch, however, the resistance value changes greatly as a result of repeated operation of pressure sensitive sheet 1, and therefore, it is necessary to detect the resistance value in accordance with this fluctuation. In addition, it is also difficult to achieve total reduction in the thickness because a certain thickness is required.